Homozygous or heterozygous

Homozygous or heterozygous twins? the differences?: these are the first questions the woman who knows she is expecting twins asks herself.

Before talking about  twin pregnancies  , it is first necessary to clarify the meaning of some terms:

  • Zygote  is the fertilized egg cell.
  • Homozygous  is a “pair of the same” fertilized egg cells.
  • Heterozygous  is a “pair of different” fertilized egg cells.
  • Homozygous twins (also called monoovular, monochorionic or monozygotic)  are individuals who are genetically identical, as they share 100% of their DNA. They are always of the same gender.
  • Heterozygous twins (also called bicorial, biovular, bizygotic, dizygotic)  are two or more brothers, not always of the same sex, who are not born from the same zygote and who genetically share only 50% of their DNA. In other words: heterozygous twins share the belly of the same mother, but not the DNA since – unlike homozygous twins which derive from the fertilization of a single egg – they are born from the simultaneous fertilization of two oocytes by two sperms.
  • Thus, depending on the case, we will have  a monozygotic or monochorionic twin pregnancy , or  a dizygotic or bichorionic or diamniotic bichorionic pregnancy

Bichorionic diamniotic twin pregnancies

As a consequence of an increasing use of  assisted fertilization techniques , in recent years there has been a general increase in the number of twin pregnancies. As we have just said, heterozygous twins do not share the same genetic heritage, so, just like in the case of two “normal” brothers,

  • they can be very different from each other,
  • they do not necessarily have the same gender (both male or female).

The expression bichorial twin pregnancy indicates that pregnancy in which two fetuses develop in two distinct gestational sacs (formed by two adherent membranes, called amnios and chorion) and in which there are therefore two distinct placentas: or more, in rare cases of multiple pregnancies.

Due to the fact that heterozygous twin pregnancies are always bichorionic, and each of the two fetuses has its own placenta as well as its own amniotic sac, they are also frequently called dichorionic (2 chorion and therefore 2 distinct gestational sacs) diamniotic (2 amniotic sacs).

The diamniotic bichorial is by far the most common twin pregnancy, and it is the one that involves the least risks: however, it still requires specific attention.

Twin pregnancies, month by month

It is usually during the  first trimester ultrasound , around the  twelfth week of pregnancy , that the woman learns that she is expecting twins, and learns whether they are monozygotic or heterozygous.

We have said that a pregnancy in which two separate ovules are fertilized by two different sperm cells is dizygotic or bichorionic.

  1. Its development,  in the  first three months, is almost identical to that of a monozygotic or monochorionic (single) twin pregnancy.
  2. Subsequently  , however, a more frequent check is needed – usually every four weeks – on how a dizygotic pregnancy is proceeding. In fact, starting from about the  twentieth week  , the pregnant woman may have some specific criticalities of her state, as is the case of a restriction on the growth of a twin.
  3. The  third trimester of heterozygous pregnancies is probably the  most challenging , both due to a greater weight gain compared to that of a single pregnancy, and due to the greater energy expenditure of the pregnant woman. While not inevitable, a woman may feel more fatigued and need more rest. The woman may also have circulatory disturbances and shortness of breath.

The most common problems of expectant mothers of twins or more

The woman who is expecting twins,

  1. in the period of gestation,  it is more likely to suffer from nausea and gastric disorders but also to develop some more serious conditions such as  gestational diabetes  and hypertension;
  2. has a non-negligible risk of a  preterm birth
  3. it is probable that then, at the end of the pregnancy, she will need  a  caesarean section ,  even if the possibility of a natural birth should not be excluded.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform, Maternicity.com.

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